The present invention relates to a water-soluble polymer composition and more in detail relates to a water-soluble polymer composition comprising a polymer or copolymer of acrylamide and methionine or a derivative thereof.
Since a polymer and a copolymer of acrylamide are water-soluble polymer of high molecular weights, the polymer and the copolymer are generally utilized as a flocculant or an injecting agent for the enhanced oil recovery.
The polymer and the copolymer of acrylamide are usually produced by a method which comprises subjecting acrylamide (and other copolymerizable monomer) to polymerization in an aqueous medium and subjecting the thus obtained aqueous gel material to hot-air drying. However, the above known method has a defect that the thermal deterioration of the polymer or the copolymer is caused in the step of hot-air drying and the molecular weight of the thus dried polymer or copolymer is reduced as compared to that before drying.
Further, in the case of using as the injecting agent for the enhanced oil recovery, such a thermally unstable polymer or copolymer suffers from the defect that the viscosity of the aqueous solution of the polymer or the copolymer is reduced by the thermal deterioration of the polymer or the copolymer because a viscous and dilute aqueous solution of the polymer or the copolymer is injected into the ground under a high pressure and the temperature of the thus injected solution is raised by terrestrial heat.
These defects of the polymer and the copolymer of acrylamide are caused by the liability thereof to be deteriorated by heat, and although various kind of thermal stabilizers has been hitherto proposed to remove the defects, any sufficient thermal stabilizer for the polymer and copolymer of acrylamide has not been proposed.
For instance, in Japanese Patent Publication No. 56-15,407(1981), the addition of a particular amino acid such as glycine, .beta.-alanine, glutamic acid, etc. to the polymer or the copolymer for preventing the thermal deterioration thereof in the step of hot-air drying. However, it is difficult to say that the thermal deterioration is sufficiently prevented. Particularly, the polymer or the copolymer of acrylamide added with such an amino acid could not satisfy the specific properties required for the injecting agent for the enhanced oil recovery.
In U.S. Pat. No. 2,917,477, a method wherein a chelate-forming compound having an amino group and/or a carboxyl group is added to an aqueous solution of a monomer to stabilize the monomer and then the thus treated monomer is subjected to polymerization has been proposed. Further, U.S. Pat. No. 3,951,934 has proposed a method for preventing the hydrolysis of the polymer of acrylamide during the polymerization by adding a compound having an amino group. However, the polymers of acrylamide obtained according to the method of above Patents are extremely insufficient in the thermal stability for using the polymer as the injecting agent for the enhanced oil recovery.
Under these circumstances, as a result of the present inventors' studies for providing a polymer or a copolymer of acrylamide having a sufficiently improved thermal stability, it has been found by the present inventors' that the addition of methionine or a derivative thereof can remarkably improve the thermal stability of the polymer or the copolymer of acrylamide, and particularly, the thermal deterioration, in the case of using the polymer composition obtained by adding methionine or a derivative thereof to the polymer or the copolymer as the injecting agent for the enhanced oil recovery, can be remarkably prevented by such an addition. And based on the findings, the present inventors have accomplished the present invention.